The present invention concerns analysis by TID (Transient Ion Drift) and has for its object an analysis device permitting carrying out measurements using this technique as well as the analysis process embodying it.
In the physicochemical industry, in particular in the field of semiconductors, it is essential to be able to detect elements or chemical compounds present in trace amounts, for example impurities present in quantities of the order of ppm (parts per million), ppb (parts per billion) . . .
Thus, for example, it is very important for the quality of produced semiconductors, to be able to detect certain impurities with a very high detection limit, particularly less than 1012 atoms/cm3.
Moreover, the importance enjoyed at present by the technology of copper interconnections in the microelectronic industry is likely to open a considerable field of application for the TID technique.
Thus, in research and development activities in microelectronics, this technique could be particularly used to evaluate the permeability of diffusion barriers. In the microelectronic industry it can serve for system controls of metallic contamination.
The TID technique used by the applicant C.N.R.S. permits detecting the element copper in trace amounts in silicon crystals with a detection limit of the order of that mentioned above. The basic principle, the theoretical modeling and experiments showing the feasibility of the use of this technique have been the object of different publications in international scientific reviews and particularly in the one entitled xe2x80x9cTransient Ion Drift Induced Capacitance Signals in Semiconductorsxe2x80x9d which appeared in PHYSICAL REVIEW B Volume 58, No. 7 of Aug. 15, 1998 and in which was cited a publication which envisages, for the first time, the use of this method for the quantitative determination of copper in silicon (Appl. Phys. Lett. 70, 3576 (1997)).
However, the TID technique as described in these different publications is not industrially usable to the extent to which the procedure for preparation of specimens, the carrying out of the test structure and the test of the physical origin of the signal are difficult or even not at all achievable on an industrial scale.
Thus, for example, the experimental techniques described in the above publications require a rapid cooling obtained by vertical drop, under the influence of gravity of the specimen into a vat containing ethylene glycol, a substance known for its high thermal conductivity.
This manner of proceeding, which can be satisfactory in the framework of an isolated measurement in a research laboratory, has many more drawbacks when transposed into the industrial field.
Thus, the high thermal stresses frequently give rise to the shattering of the analyzed specimens, the delicate handling necessary and the size limit to small specimens (less than about 1 cm), as well as the use of a specific chemical cooling liquid such as ethylene glycol, are conditions which are not adapted to systematic or automated use of this technique in industry, for example, in an environment with a controlled atmosphere (clean room) and can have risks for the workers and/or the environment.
A second source of difficulties which renders the TID technique unusable in the industrial field, arises from the production of the test structure itself. Thus, the TID analysis requires a metal/semiconductor rectifier interface to carry out capacitative measurement. In research laboratories, this structure can be obtained by vacuum evaporation of a metallic layer of aluminum on silicon.
Here again, the disadvantages which result are not tolerable in an industrial context.
Thus, the excessive duration necessary for the preparation of the specimen (about two hours for an aluminum deposit of 100 xcexcm by chemical deposition in vapor phase) decreases productivity and, more seriously, has the result of a loss of sensitivity of the technique. The TID signal decreases after thermal quenching with a time constant of several hours by virtue of the precipitation of interstitial copper which is thus not available or sufficiently mobile to contribute to the measurement signal.
Similarly, the alteration of the specimen by deposition of a metallic layer which cannot, in the best cases, be ultimately removed other than by chemical attack, requires a supplemental treatment that is costly in time and money and which is damaging to the specimen itself, which influences negatively the quality of the surface of the tested specimen (destructive or semi-destructive method).
Finally, the exploitation, which is to say the interpretation properly so-called of the analysis signal obtained in the laboratory by the use of known devices, is no longer satisfactory for industrial application.
Thus, the signals obtained by the TID technique are electrical signals which are differentiated from the signals due to other phenomena (emission of free charge carriers) only by the temporal shape of the electrical excitation which gave rise to the signal.
In studies carried out in the laboratory, previously published and described in the mentioned document, it has been suggested to measure the amplitude of the signal as a function of the duration and the height of the electrical impulse. The corresponding evolution is thus characteristic of the physical process (ionic drift or emission of electrical carriers) which is at the origin of the signal.
This procedure is however too long to be of practical use in industry and is not susceptible to being automated.
The present invention has particularly for its object to overcome these drawbacks.
To this end, it has for its object a device for the quantitative determination of copper in silicon by transitory ion drift (TID) comprising essentially a heating means and a rapid cooling means for the specimen to be analyzed, an electrode for measuring the electrical capacity of the specimen as well as a unit for generating an excitation signal and for processing the electrical measuring signal, characterized in that:
the heating means for the specimen to be analyzed consists in at least one halogen lamp,
the rapid cooling means for the specimen to be analyzed is a water cooler, and
the electrode for measuring the specimen to be analyzed is a mercury electrode.
The invention also has for its object a process for the quantitative determination of copper in silicon by transient ion drift (TID) using a device according to the invention and characterized in that it comprises the steps consisting in:
introducing the specimen into the thermal measuring chamber,
reheating the specimen to a temperature comprised between about 900 and 1000xc2x0 C., for a time comprised between about 30 second and 3 minutes, preferably at 950xc2x0 C. for 2 minutes thanks to the halogen lamp heating means,
abruptly cooling the specimen by the rapid water cooling means,
carrying out the measurement by means of the mercury electrode and by a specific electrical signal generated by the unit for generation and treatment of electrical signals, and
using the obtained result in the form of a curve or a table of data.